1. Field of the Invention
This invention relates to a method of producing high magnetic flux density grain oriented silicon steel sheets having excellent iron loss properties, and more particularly relates to a method of producing grain oriented silicon steel sheets having a thickness of about 0.10 to 0.25 mm in which iron loss is significantly and advantageously improved without decreasing magnetic flux density.
2. Description of the Related Art
Grain oriented silicon steel sheets are mainly used as iron cores for transformers and other electrical devices, and accordingly need to have excellent magnetic characteristics, particularly low iron loss as exemplified by the W.sub.17/50 value.
It is important for this purpose to highly align the &lt;100&gt; orientation of the secondary recrystallized grains in the steel sheet in the rolling direction. It is also necessary to decrease the amounts of impurities and precipitates in the steel as much as possible. In consideration of the above requirements, iron loss has been improved from year to year. In recent years a thickness of 0.23 mm in a sheet product having a W.sub.17/50 of 0.90 W/kg or less has been obtained.
However, the demand strongly tends to request electrical machinery and apparatus having still further reduced power loss in view of the current energy crisis. For this purpose, it is important to develop grain oriented silicon steel sheets having much lower iron loss as a core material than ever before.
In general, many fundamental techniques are known for reducing the iron loss of grain oriented silicon steel sheets. These include metallurgical methods such as increasing the Si percentage, thinning the sheet product, finely dividing the secondary recrystallized grains, reducing the amounts of impurities, highly aligning the secondary recrystallized grains of the ( 100 ) [100] orientation, and the like.
However, with regard to the above methods, increasing the percentage of Si is unsuitable for industrial production processes because an Si content over 4.5 wt % significantly deteriorates cold-rolling workability.
On the other hand, various methods have been proposed such as thinning the sheet product. For example, Japanese Patent Laid-Open Nos. 58-217630 and 59-126722 disclose a method in which Sn and Cu are added to a grain oriented silicon steel sheet containing AlN as an inhibitor to obtain a product having a thickness of 0.15 to 0.25 mm. Japanese Patent Laid-Open Nos. 62-167820, 62-167821 and 62-167822 disclose a method in which the average grain size of a grain oriented silicon steel sheet containing MnSe and MnS as inhibitors is adjusted to the range from 1 to 6 mm after secondary recrystallization to obtain a product having a thickness of 0.15 to 0.25 min.
However, addition of Sn and Cu to a grain oriented silicon steel sheet containing AlN as a main inhibitor produces a relatively high magnetic flux density, but produces a W.sub.17/50 value of 0.85 to 0.90 W/kg. As shown in Table 5 of Japanese Patent Laid-Open No. 59-126722, this cannot be said to be a satisfactory value. Moreover, it is disadvantageous that since the proper rolling reduction of final cold-rolling exceeds 80% in the production of a grain oriented silicon steel sheet containing AlN as a main inhibitor, when the thickness ,of the product sheet-is decreased, secondary recrystallization becomes unstable, and the probability of producing a favorable iron loss is significantly decreased.
On the other hand, the method of thinning grain oriented silicon steel sheets containing MnSe and MnS as inhibitors and decreasing the sizes of the crystal grains therein causes the magnetic flux density to be inferior to that of the grain oriented silicon steel sheet containing AlN as a main inhibitor, but this method is excellent in regard to refining the crystal grains. Therefore, this method provides an improved iron loss, for example, a W.sub.17/50 of 0.83 to 0.88 W/kg shown in Table 2 of Japanese Patent Laid-Open No. 62-167820. However, it cannot be said that the level of the iron loss value is a satisfactory value. This method also has the problem of instability in producing materials with low iron loss.
Further, in order to highly align the (110) [001] orientation of the secondary recrystallized grains, it is necessary to carry out rapid secondary recrystallization while sufficiently inhibiting the growths of normal grains. Adding Cu to steel is well known as a method of increasing inhibition. For example, Japanese Patent Publication No. 48-17688 discloses the technique of increasing inhibition by adding 0.10 to 0.30% Cu and moving MnTe to the grain boundaries. Japanese Patent Laid-Open No. 50-15726 discloses the technique of relaxing the limitation of hot-rolling conditions related to the precipitate of an inhibitor by decreasing the melting temperature of the inhibitor in slab heating by using manganese copper sulfide containing 0.1 to 0.5 % Cu as an inhibitor. Japanese Patent Publication No. 54-32412 discloses the technique of improving magnetic flux density by adding 0.2 to 1.0 % Cu or Ni so as to rationalize the rolling reduction and final finish annealing. Japanese Patent Laid-Open No. 61-12822 discloses the technique of increasing inhibition by adding 0.02 to 0.20 % Cu and precipitating (Cu, Mn).sub.1.8 S fine grains as an inhibitor, thereby improving the magnetic properties. Japanese Patent Publication No. 54-32412 discloses that a very high magnetic flux density and favorable iron loss can be obtained by adding both Cu and Sb to a grain oriented silicon steel material and secondarily recrystallizing the material at 800.degree. to 950.degree. C.
The effect of addition of Cu to steel is caused by the function of the inhibitor in steel to increase the inhibition. This is due to the fine dispersion and precipitation resulting from the change to Cu.sub.2-x Se of the type of the inhibitor precipitated and the control of deterioration in the inhibition in a steel surface portion. The deterioration of inhibition in the steel surface portion is the most critical problem in actual production processes in factories. The addition of Cu to steel is extremely effective because it can avoid deterioration and maintain the inhibition in the surface layer.
The demand for increasing inhibition in the surface layer of a steel sheet is increased with a decrease in the thickness of the steel sheet. The use of Se as an inhibitor element in place of S and the addition of Sb to steel are known as means for supplementing the function of the Cu addition and increasing inhibition. Namely, the use of Se causes the precipitates of CuSe as an inhibitor which is more stable than a CuS as an inhibitor with respect to the decomposition of the inhibitor in the surface layer of the steel sheet, thereby maintaining inhibitor in the surface layer of the steel sheet. The addition of Sb causes segregation of Sb on the surface of the steel sheet and segregation around Cu.sub.2-x Se, thereby further inhibiting the decomposition of Cu.sub.2-x Se used as an inhibitor.
Since the use of Se as an inhibitor element in place of S and the addition of Sb are effective for preventing the deterioration in inhibition in the surface layer portion of the steel sheet, as the addition of Cu, they are generally used in the industrial field.
However, it was found that there is a problem that the effects of Cu addition deteriorate as the thickness of a sheet material is further decreased. This makes it difficult to comply with the strong demand for further decreasing the iron losses. For example, Japanese Patent Laid-Open No. 61-159531, discloses embodiments of steel plates each containing 0.04 to 0.19 % Cu and respectively having a final thicknesses of 0.225 and 0.175 mm. However, the embodiments respectively have magnetic flux densities and iron losses which are B.sub.10 =1.87T (B.sub.8 =about 1.85T) and W.sub.17/50 =0.94 W/kg and B.sub.10 =1.88T (B.sub.8 =1.86T) and W.sub.17/50 =0.90 W/kg. The effect of the decrease in the thickness of each of the steel sheets was not sufficiently obtained.